Tectonic Evolution of the Amdo Terrane, Central Tibet: Petrochemistry and Zircon U-Pb Geochronology

Abstract

The formation and evolution of the basement rocks in the Tibetan Plateau remain poorly constrained. Here we report petrology, geochemistry, and zircon U-Pb geochronology of orthogneisses, metamafic intrusions, and metasedimentary rocks from the Amdo basement in central Tibet. The oldest Neoproterozoic (∼820 Ma) orthogneisses show chemical affinity to a volcanic arc setting. These rocks, together with the coeval arc-magmatic rocks, belong to an Andean-type setting traced from the Seychelles, Madagascar, western India, Lhasa, and South China blocks, following the Rodinia supercontinent assembly. The later early Paleozoic (∼500 Ma) bimodal intrusions in Amdo also show an affinity to arc-related igneous rocks and probably formed in an active continental margin associated with the subduction of the Proto-Tethys Ocean beneath the Gondwana supercontinent. In contrast to the adjacent Lhasa and Qiangtang terranes, the Amdo block experienced complex Neoproterozoic and early Paleozoic tectono-thermal events and coherent Jurassic (∼190 Ma) high-pressure granulite-facies metamorphism and associated magmatism, suggesting that the Amdo block was an isolated microcontinent in the Tethyan Ocean and underwent deep subduction before its collision with the Qiangtang or Lhasa terrane.